The present invention relates to a hydraulic pressure control system for controlling shocks occurring upon engagement of a torque transmitting frictional element that is engaged to effect shifting between gear positions in an automatic transmission.
Vehicular automatic transmissions are known wherein in response to the vehicle operating conditions shifting between gear positions is effected automatically by supplying hydraulic fluid pressure to a selected one or ones of frictional elements. U.S. Pat. No. 4,502,354 (corresponding to JP-A-57-47056) discloses a hydraulic pressure control system wherein in order to reduce shift shock during a change in gear positions, a frictional element actuating hydraulic pressure (a line pressure) is maintained equal to the magnitude of a target hydraulic pressure for a predetermined holding time. According to this known control system, the magnitude of the target hydraulic pressure is determined as a function of an engine load signal, viz., a throttle opening, so that if the throttle opening fails to represent the magnitude of an input torque to the transmission after a long use of the engine, substantial shift shock will occur. Besides, if the engines different in engine torque for the same throttle opening are used, the same hydraulic control system can not be used in association with the different engines.
An object of the present invention is to provide a hydraulic pressure control which can be used in association with different engines having different output vs. throttle opening characteristic curves.